Hong Kong's leader warned on Tuesday that violence will not be tolerated, a day after authorities arrested 10 people and seized suspected explosives ahead of a crucial vote on a China-backed electoral reform package this week.

Security has been stepped up across the Chinese-ruled city, including at government buildings and train stations, as it braces for a fresh showdown over plans for how its next leader is elected in 2017.

Authorities are taking no chances after mass pro-democracy protests crippled parts of the former British colony late last year and presented China's Communist Party leadership with one of its biggest political challenges in decades.

"Hong Kong society should not tolerate any illegal activities. Whether these are violent or non-violent, we should not allow any illegal activities to be justified," Chief Executive Leung Chun-ying said in a televised briefing ahead of the weekly Executive Council meeting.

Ten people were arrested on suspicion of conspiracy to manufacture explosives, police said on Monday, adding that some belonged to a radical group. They did not elaborate and no charges have been laid.

Hong Kong's legislature is due to begin debate on the electoral reform package in the Legislative Council on Wednesday, with a vote due by the end of the week. Pro-democracy protesters are staging evening rallies throughout the week.

Beijing has proposed a direct vote for Hong Kong's next leader in 2017, but only from among pre-screened, pro-Beijing candidates. Democracy activists say they want a genuinely democratic vote.

"If the pan-democrats stubbornly insist on vetoing the proposal, democracy in Hong Kong will come to a standstill," said Song Ru'an, a Chinese Foreign Ministry official in Hong Kong.

The head of China's Hong Kong Macau Affairs Office, Wang Guangya, in an interview with two pro-Beijing newspapers, reiterated Beijing's desire to see the electoral package passed. He said that if the electoral framework were accepted in its current form, there would still be room for changes in future, though he gave no specifics.

With tensions running high before the debate, Hong Kong's Independent Commission Against Corruption has also said it is investigating allegations by an unidentified legislator that he was offered a bribe to vote for the package.

Some of those arrested in the raids belonged to a little-known group called the National Independent Party, Hong Kong media reported on Tuesday. According to its Facebook page the group was set up in January, but the page has now been deleted.

A June 1 post purportedly from the group warned that, if the reform package was passed, "Hong Kong people should be mentally prepared there will be casualties".

The Global Times, a widely-read tabloid published by the Chinese Communist Party's official People's Daily, wrote in an editorial that following the finding of the explosives, Hong Kong risked descending into chaos.

"The people of Hong Kong should really pay attention to the impression that outsiders have of it becoming more and more chaotic. This is related to the world's confidence in Hong Kong," it said.

Rising tensions have resulted in a new front of radical activism in Hong Kong, where some groups have staged small but disruptive protests targeting mainland Chinese visitors.

Monday's raids by scores of officers rattled some legislators and residents. Posts on social media questioned the timing of the arrests, details of which were leaked to Hong Kong media before an official announcement.

Others were quick to sound a cautious note.

"I suggest we look very carefully and calmly at this case before we afford this incident too much priority or seek to amend the Hong Kong threat profile," said Steve Vickers, chief executive of risk consultancy SVA and former head of the Royal Hong Kong Police Criminal Intelligence Bureau.

Britain handed Hong Kong back to China in 1997 under a promise that core personal and commercial freedoms, backed by a British-style legal system, would be protected for 50 years.

Two senior police officers have told Reuters at least 5,000 officers will be standing by on the day of the vote. Authorities expect the scale of protests to be much larger, and possibly violent, if the reform package is passed.